•The receptor molecule-hormone complex enters the nucleus, seeks out a specific region of the DNA molecule, and initiates the synthesis of mRNA.

•The newly formed mRNA codes for the formation of specific enzymes that will accomplish the desired result.

The presence of most hormones also elicits a vascularly mediated negative feedback response, in that subsequent to a desired response, the further production and/or release of that particular hormone is inhibited.

PITUITARY GLAND

The pituitary gland (hypophysis) is composed of several regions, namely, pars anterior (pars distalis), pars tuberalis, infundibular stalk, pars intermedia, and pars nervosa (the last two are known as the pars posterior) (see Table 10-1 and Graphic 10-1).

Since the pituitary gland develops from two separate embryonic origins, the epithelium of the pharyngeal roof and the floor of the diencephalon, it is frequently discussed as being subdivided into two parts:

•the adenohypophysis (pars anterior, pars tuberalis, and pars intermedia) and the

•neurohypophysis (pars nervosa and infundibular stalk).

The pars nervosa is continuous with the median eminence of the hypothalamus via the thin neural stalk (infundibular stalk).

The pituitary gland receives its blood supply from the right and left superior hypophyseal arteries, serving the median eminence, pars tuberalis, and the infundibulum, and from the right and left inferior hypophyseal arteries, which serve the pars nervosa.

Hypophyseal Portal System: The two superior hypophyseal arteries give rise to the

•primary capillary plexus located in the region of the median eminence.

•Hypophyseal portal veins drain the primary capillary plexus and deliver the blood into the secondary capillary plexus, located in the pars distalis.

•Both capillary plexuses are composed of fenestrated capillaries.

Pars Anterior

The pars anterior is composed of numerous parenchymal cells arranged in thick cords, with large capillaries known as sinusoids, richly vascularizing the intervening regions. The parenchymal cells are classified into two main categories: those whose granules readily take up

E N D O C R I N E S Y S T E M 231

stain, chromophils, and those cells that do not possess a strong affinity for stains, chromophobes.

•Chromophils are of two types: acidophils and basophils. Although considerable controversy surrounds the classification of these cells vis-à-vis their function, it is probable that at least six of the seven hormones manufactured by the pars anterior are made by separate cells (see Table 10-1).

Hormones that modulate the secretory functions of the pituitary-dependent endocrine glands are somatotropin, thyrotropin (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), interstitial cell stimulating hormone (ICSH), prolactin, adrenocorticotropin hormone (ACTH), and melanocyte-stimulating hormone (MSH).

It is believed that two types of acidophils produce somatotropin and prolactin, whereas various populations of basophils produce the remaining five hormones.

•Chromophobes, however, probably do not produce hormones. They are believed to be acidophils and basophils that have released their granules.

Control of Anterior Pituitary Hormone Release:

•The axons of parvicellular, hypophyseotropic neurons whose soma are located in the paraventricular and arcuate nuclei of the hypothalamus terminate at the primary capillary bed.

These axons store releasing hormones (somatotropinreleasing hormone, prolactin-releasing hormone, corticotropin-releasing hormone, thyrotropin-releasing hormone, and gonadotropin-releasing hormone) and inhibitory hormones (prolactin-inhibiting hormone, inhibin, and somatostatin).

The hormones are released by these axons into the primary capillary plexus and are conveyed to the secondary capillary plexus by the hypophyseal portal veins.

The hormones then activate (or inhibit) chromophils of the adenohypophysis, causing them to release or prevent them from releasing their hormones.

•An additional control is the mechanism of negative feedback, so that the presence of specific plasma levels of the pituitary hormones prevents the chromophils from releasing additional quantities of their hormones.

Pars Intermedia

The pars intermedia is not well developed. It is believed that the cell population of this region may have migrated into the pars anterior to produce melanocyte-stimulating hormone (MSH) and adrenocorticotropin. It is quite probable that a single basophil can produce both of these hormones.